Here is the verbal compilation of the entire Collins 20V-3 project.
Pictures to go with the descriptions and illustrations to follow.
A lot of people in the 3870 group in the Northwest contributed to the final successful completion of this project.
A few people are mentioned in the article but by no means everyone. To those people, Thank You !.
Any questions, please email [email protected]
Putting the Collins 20V-3 onto 75 meters
I have one of the most interesting jobs on the planet. While I must travel quite a bit, these travels put me into contact with many fellow hams throughout the country. I have sent many a boatanchor home via the airlines as luggage.
In the Pacific Northwest we have a very active AM boatanchor group which meets daily on 3870 khz. Over the last few years, several of the “gang” have put on the air broadcast transmitters from Gates, Collins, Bauer and others. This put a desire in my gut to try my hand . . . but how would I ever find one. As fate would have it, while on one of my trips to Oregon, I happened to pass the studios of a station near Portland. I saw someone in the parking lot and stopped to chat. Low and behold, I asked the owner if he possibly had a surplus broadcast transmitter he might be looking to get rid of. I was shocked when he said he did. I almost fell over. He had a Collins 20V-3 that had taken a close lightening strike and instead of fixing it, he just purchased a new solid state rig.
With the help of Ray, W7GEC and Steve, WB7BNZ, we took off to Oregon in the early morning of October 2002. 2 ½ hr later we were at the site (pics)
I’m sure anyone that has done this has his preferred methods of extraction but the following are the steps we took which proved to be quite satisfactory. Getting a transmitter out of a transmitter shed is a big undertaking. If you are lucky, the building was built and then the transmitter put in it. Sometimes it is the other way around.
First unhook all the power. In my case it already had been. Then take out all of the tubes and all heavy iron in the bottom of the unit. This usually will include the plate transformer, modulation reactors, modulation transformer and several other heavy pieces. The goal is to make the transmitter as light as possible so it can be maneuvered. Next making sure you have a heavy duty piano type dolly, tip the transmitter over onto the dolly and wheel it out and up a ramp into an awaiting truck. Sounds easy but there was a lot of groaning but it did go in as planned. Thanks to Steve’s experience in doing this several times, we were able to get the job done fairly quickly. We then drove 2 1/2 hours back to Seattle and started the unloading process.
I have to thank Ray for making the supreme sacrifice. As the transmitter came out of the pickup, it fell a short distance onto the ramp. One of Ray’s fingertips was caught underneath as it came out. That fingernail was never the same. All 1250 lbs minus a little heavy iron hammered one fingernail. He’s fond of saying that he sacrificed a fingernail to advance my AM enjoyment.
Step 1 - Cleanup
First thing I did was vacuum out all of the mouse “stuff” and then spray engine cleaner all over the now bare inside bottom to get rid of all of the grease and gunk that had accumulated over the years. You have to be very careful with this material as its very toxic so put on gloves and eye protection . . . and don’t breath in the stuff. It does do a great job, though.
Next I went out and bought large steel rollers that could be attached under each corner so that the unit can be moved around. These we purchased at a local salvage yard that also carries new tools, pulleys and rubber wheeled rollers. You may need to cut corner plates out of heavy iron and fasten under the corners to hold the rollers and all of the weight of the transmitter. The attachment can be done by tipping the cabinet on one side, roping it so it doesn’t tip all the way over, and hand drilling the holes needed to fasten two rollers on. Then when the two are on, block that side so the rollers don’t move and then tip the cabinet the other direction to put the other two. If you’ve got several people and the right dolly you could probably put it on its side flat and drill and attach the rollers in one step.
After the rollers were all on, I then reassembled all of the heavy iron and capacitors that had been removed by Steve WB7BNZ. Steve had been emphatic about marking each and every connection/wire so re-assembling wasn’t too difficult. (pics) I also took some color pictures before we started removal back in Oregon– I’d really recommend that as I ended up with one extra short wire. In addition, two wire labels fell off but after looking at the picture I could tell exactly where it went. It was now December and it got cold in the shop so not much got done till late spring.
I’ll go about explaining actual revisions in the next issue.
In the first article I expounded on how the Collins 20V-3 got to my QTH and what I did to get it to the point of modifying it for 75 meters. Yes 75 meters. Most people will change a broadcast transmitter to 160 meters but 160 is so noisy in the city that I chose to go all the way to 75. Changing it to 75 meters is quite easy. There are four areas that have to be modified. 1) the oscillator coil 2) the buffer coil 3) crystal 4) tank circuit.
The 20V family of Collins transmitters were used all over the country. They are well built and look great. The 20V and 20V-2 look very similar. The 20V-3 is essentially the same transmitter but with many additional circuits added for fail safe operation. Even among 20V-3’s there were several tube lineups used in the modulators. Mine has a pair of 4-400’s driving a pair of 4-400’s in the PA. Mine also originally had a pair of rare 575 hv rectifier tubes. Many 20v’s used 872’s in the hv area. The original owner was aware that the filament transformer was overworked supplying filament voltage for the 4-400’s and hv and mv rectifiers. He therefore replaced both hv and mv with solid state replacements. This makes things work much cooler and causes less strain on the filament transformer albeit a few less firebottles to get glassy eyed over.
At this point I’ve also got to acknowledge the expert advice and materials supply by Jerry, K7LFE. Jerry brought back to life his own 20V so a lot of the calculations throughout my refurbishing job had already been made by him. He walked me through the rewinding of the plate choke, oscillator and buffer coils as well as initial course tuning of the tank circuit. The previously mentioned lightening strike destroyed the plate choke. You would need to replace this to operate on 75 meters anyway. I was able to rewind the new one onto the old plate choke form. I cut all of the old wire off the ceramic form and wound 33 turns of #22 wire on it in one layer. That’s it. See figure B.
Next I modified the oscillator and buffer coils. This information all came from the efforts of Jerry K7LFE. Both are done exactly the same. (See diagram) Take off the old wire from the coil forms and wind new ones according to the chart. It’s pretty easy. These coils were modified for 75 meters so you’d need another set for each other band you chose to operate on. Obtain some old 1625 tubes and salvage the tube base to make your own oscillator and buffer coils. Pins configuration is the same.
Next comes the tank circuit. This is probably the step that takes the longest. The stock 20V series come with both a pi and an L tank circuit. Jerry has pounded one thing into my brain over the years and that is “make things simple”. That meant a simple pi network was the tank of choice. I cleared out everything in the tank compartment. Put the the loading and plate caps aside and stored everything else. I couldn’t bring myself to hack up the big fixed capacitor or the large loading coil so I put them nicely away. If you wanted to go on 160 meters then you might be able to retain these and just tap the coil differently. (add pic) I then put in the new plate choke and hooked the choke bypass capacitor up at the base. Then I put in the new tank coil I had made by cobbling together a commercial coil and about 8 turns of copper tubing. (add pic) It isn’t pretty but seems to work. There’s nothing magic about this. Just look at any modern rf amplifier pi network and copy its construction. Use the guidelines in the handbook. I was able to retain the original pate tuning cap as well as the loading cap. A large transmitting mica cap of 500 pf needed to be added to increase the capacitance of the loading cap. Then put an RFC choke to ground from the loading capacitor to make sure the transmitter pops the breakers if there is a blocking capacitor failure. This is always a good safety precaustion. Make sure that a good ground is attached to the transmitter and a dummy load attached to the output. Use a grid dipper to confirm that the tank is resonant where you want it.
Next look over the transmitter from head to toe to look for burnt resistors, melted wires, transformers leaking goooo and capacitors that might be burned. It’s always better if the unit was simply taken out of service because of obsolescence rather than by some big circuit fault, like lightening. Try getting in touch with the engineer who probably babied this transmitter for years to find out as much as possible of its history. It will lead you to known problems if there had been any.
As previously described in ER _ _ _ take out all of the hi and med voltage rectifiers. Turn on the filaments and check for smoke. Then turn on the Plate switch to see if the proper modulation current is present. Then look at the other meter readings to see if they are in the proper range. This will also tell you if the oscillator section is functioning. Listen on a nearby receiver to find the signal. You are now ready to re-attach the med and hi voltage rectifiers. With one finger on the plate vswitch and one finger on the plate off switch, push in the “on” switch and confirm that you have hi-voltage. Mine didn’t, but only because the back door was ajar and the safety switch worked as advertised. There have been many stories about people who “wire around” the safety switches in order to keep the doors open for tuning. DO NOT succumb to this temptation. Its a much used expression but with this type of voltage, you don’t get a second chance to make a mistake. Figure out another way to do take those readings.
I added another whole level of difficulty to using my 20V-3. I wanted the transmitter to stay in the outdoor shop and remote it from my office on the second floor. In addition I didn’t want to have to go out to the transmitter any time I needed to change frequency so a VFO was needed, remotely tuned, of course.
Remote switching of the transmitter on and off.
There are various schemes of doing this but the one I chose was from the “KISS” method. Keep It Simple Stupid. The 20V-3 has a feature whereby you can turn it on and off remotely by using only two momentary switches. One normally off and the other normally on. Adding another momentary switch can also control the Plate Off switch. Pressing the Plate On switch cycles it from lighting the filaments, waiting a selected amount of time and then applying the HV. Once the HV comes on you are putting a carrier on the air so I immediately push the Plate Off switch to shut off the HV. I use these two switches to “PTT”. Then when I want to kill the transmitter altogether I hit the Filament Off switch and the whole thing shuts down. Pretty simple. I know several other people have come up with some real ingenious schemes using digital techniques. I’m sure there are other ways of doing this like diode directed relays but I kept it simple. It works.
I also have a separate antenna for 75 meters that stays connected to the 20V-3 all the time. The coax from the 75 mtr antenna used to come all the way back from the antenna to the shack but I “broke it” right at the 20V-3. So I have a piece of RG-8 from there back to the operating position. I’ll show you how I utilized that in a minute. I use a full wave loop antenna as the receiving antenna feed with open wire. I use that antenna for operation on all other bands. One caution in using a separate receiving antenna. Make sure you not only mute the receiver but also short the antenna to ground when the “Big Iron” transmitter is on. I roasted an RF input coil because the antenna wasn’t grounded, only the B+ was turned off. I failed to think that one through. So now we’ve got the transmitter able to turn “Plate On” and “Plate Off”.
Now for the VFO. I had a WRL 755 VFO available with its own power supply so I used that. I now utilize that piece of coax from the 20V-3 back to the shack that remained. One end would go to the VFO in the shack and the other to the 20V-3. I found that just putting it in the crystal socket didn’t get me any oscillation. After talking it over with several people in our NW group, especially WA7NAN, Bob, he pointed out that I already had oscillation in the VFO. Therefore I shouldn’t use the 6AU6 in the 20V-3 as another one but rather as a simple amplifier. To do that I unhooked the crystal socket entirely and put the VFO signal directly to the grid of the 6AU6 through an added blocking capacitor. It worked some of the time so I then decided that there just wasn’t enough signal getting to the 20V-3 so I made a simple broadband amplifier, see figure 5, that I used to boost the VFO signal and it worked perfectly. Now I had a remote VFO for the 20V-3 but no way to turn it off and on when the transmitter was on or off. I needed an open/close circuit to do that. A relay at the VFO could do that just fine but how would I power it to coincide with the transmitter cycling? I had suggestions from many people but finally Larry, W , drew up a clever circuit that activated a 220v relay in parallel with the existing “plate on” actuator at the transmitter. Here’s hwere it gets clever. He used a battery at the transmitter that had one end grounded and put the V+ end onto the same coax through an rf choke that the VFO signal was using. Make sure there is a capacitor blocking the DC front entering the 6AU6 grid. At the small broadband amplifier end, another RF choke was added that brought that V+ off of the coax to close a simple 9v relay that turned the VFO output on and off. Make sure there is another capacitor to block the DC from getting into the broadband amp. It worked perfectly.
There you have it. A complete remote system for the 20V-3 including a VFO. This relay in the VFO will also be used to turn on and off any number of other controls to mute the receivers a well as ground the antennas.
Thanks to all the gang on 3870 in the Pacific NW for their support and patience. To look for our activities on our website go to my website at www.qsl.net/k7rld and then take the link to the NWAM webpage. Thanks to Al Waller for his website hosting. You can also see many more pics of the transmitter extraction process.
Don’t think you’ve got to be an engineer to make one of these babies play. Common sense and some guidance from ham friends can make it happen for you.
73’s - John